1. Field of the Invention
The present invention relates to a semiconductor device. In particular, it relates to a semiconductor device using copper interconnects, which has bonding pads made of aluminum.
2. Description of the Prior Art
In semiconductor integrated circuits beginning with the sub-quarter micron generation, demands have been made for alternative interconnect materials to replace the conventionally used aluminum-based interconnect. Accompanying the miniaturization of semiconductor integrated circuits, interconnect caused delay time has been increasing in comparison with the delay time due to the transistor elements. Moreover, accompanying the miniaturization of interconnect width, interconnect resistance has increased. The increase in interconnect resistance invites electric potential on the power supply line to fall and clock signal delay time to fluctuate, causing malfunctions to occur. In addition, since the density of the electric current flowing through an interconnect increases, adverse influences on reliability against electromigration have become more acute. It is difficult to deal with these problems using aluminum-based interconnects.
Copper has shown great promise as an interconnect material to be used for semiconductor integrated circuits from the sub-quarter micron generation forward. Copper is characterized by low resistance and is highly resistant against electromigration. These characteristics of copper are favorable for use in semiconductor integrated circuits from the sub-quarter micron generation forward.
When copper is used as the interconnect material, it can be generally considered that the bonding pads are also formed with copper.
However, since bonding pads formed with copper easily oxidize, problems caused by oxidation of the bonding pads may occur. Copper is a material that oxidizes very easily. In addition, when exposed to the outside atmosphere, oxidation is accelerated through the moisture in the air. The surface of the bonding pads formed with copper is easily oxidized. Once the surface of the bonding pads is oxidized, sufficient adhesive strength between the wiring and the bonding pad cannot be obtained where the wiring is bonded. Moreover, the copper oxidation does not stay at the surface layer, but progresses deep inside the copper. Oxidization of bonding pads formed with copper begins at the exposed portions not covered with a bonding ball, and the corrosion of the bonding pads then progresses. Ultimately, the entire bonding pad may be corroded. If the corrosion progresses deeper, the copper interconnect connected to a bonding pad may be corroded.
To deal with this problem, a semiconductor device comprising bonding pads formed with aluminum on top of the copper interconnect is disclosed in Japanese Patent Application Laid-Open No. Hei 11-135506. In this well known semiconductor device, as shown in FIG. 13, a copper interconnect 504 is formed on the upper surface side of a silicon substrate 501. The copper interconnect 504 is covered with an insulation/protection film 512. In the insulation/protection film 512, an aperture 512a, which reaches the copper interconnect 504, is provided. On top of the copper interconnect 504, an aluminum film 510 is formed. The aluminum film 510 is connected to the copper interconnect 504 through the aperture 512a. The aluminum film 510 is used as bonding pads.
FIGS. 14A through 14C illustrate a method for manufacturing such a well-known semiconductor device. Referencing FIG. 14A, a silicon oxide film 602, which acts as an interlayer film, is formed through a CVD method on top of a silicon substrate 601 whereupon transistors are formed. Typically, the film thickness of the silicon oxide film 602 is approximately 1 μm. Afterwards, a photolithography technique and a dry etching technique are used to form a trench 602a with a depth of 50 nm. The depth of the trench 602a is 550 nm. In addition, each of the apertures, which respectively reach the source, drain, and gate of a transistor formed on the silicon substrate 601, are formed. However, the formed apertures are not shown in FIG. 14A. Moreover, a titanium nitride film and copper film are sequentially formed through a CVD method. The titanium nitride film prevents diffusion of the copper film, and also improves the adhesiveness between the copper film and the silicon oxide film 602. The film thickness of the formed titanium nitride film and copper film are 50 nm and 500 nm, respectively. In addition, the portion of the formed titanium nitride film and copper film besides the portion that is within the trench 602a are removed through a chemical mechanical polishing (CMP) method. As shown in FIG. 14A, a titanium nitride layer 603 and copper interconnect 604 are formed.
Then, referencing FIG. 14B, a silicon nitride film 605, silicon oxide film 606, silicon nitride film 607, and silicon oxide film 608 are sequentially formed. The silicon nitride film 605 prevents the diffusion of copper from the copper interconnect 604. A trench 608a is formed, as shown in FIG. 14B, on the silicon nitride film 607 and silicon oxide film 608. A trench 606a, which reaches the copper interconnect 604, is formed within the trench 608a. 
Then, referencing FIG. 14C, a titanium nitride film is formed through a CVD method. The thickness of the titanium nitride film is 50 nm. The formed titanium nitride film is etched back through an anisotropic etching. The titanium nitride film is not completely removed but remains at the sidewall of the trench 608a, thereby forming a titanium nitride layer 609. Similarly, the titanium nitride film is not completely removed but remains at the sidewall of the trench 606a, thereby forming a titanium nitride layer 610.
Between formation of the silicon nitride film and formation of the titanium nitride layers 609 and 610, the surface of the copper interconnect 604 is exposed, and a copper oxide is formed. In addition, on the surface of the copper interconnect 604, deposited material remains after etching back the titanium nitride film. Consequently, after forming the titanium nitride layer 610, the copper oxide and deposited material formed on the surface of the first layer copper interconnect 604 are removed by using O2plasma, diluted hydrofluoric acid, and hydrogen (hfac) gas.
Moreover, a copper film is formed through a CVD method. The portions other than the portions within the trenches 606a and 608a of the formed copper film are removed through a CMP method, to form a copper interconnect 611 as shown in FIG. 14C. Continuing, as shown in FIG. 15A, an insulation/protection film 612 is formed with silicon nitride. Moreover, an aperture 613 is formed using a lithographic technique and a dry etching technique.
Continuing, the copper oxide formed on the surface of the copper interconnect 611 is removed by using O2 plasma, diluted hydrofluoric acid, and H (hfac) gas. Moreover, an aluminum film is formed using a sputtering method. The aluminum film is patterned through a lithographic technique and etching technique to form an aluminum bonding pad 614.
However, in the well-known semiconductor device manufacturing method, the aluminum film formed on the top surface side of the second layer copper interconnect 611 is used only for forming the bonding pad. More effective utilization of the formed aluminum film is desired.